The present invention generally relates to the use of Superconducting Quantum Interference Devices (SQUIDs) and, more specifically, to sensors for use with SQUIDs in magnetometry and gradiometry.
Superconducting Quantum Interference Devices (SQUIDs) are widely used to detect and quantify extremely weak magnetic signals, such as those emanating from the brain of a human or an animal. In magnetoencephalography, brain wave signals ranging from 10 to 10.sup.3 femto Teslas (fT) are measured. Other uses include near magnetic field antennas, high sensitivity magnetometers, and non-destructive material evaluators.
Because of their great sensitivity, SQUID magnetometers are subject to having the signal of interest swamped by large noise backgrounds. Contributors to these noise backgrounds include the earth's magnetic field, and cultural noise such electrical power lines and cords, and automobiles. For example, the natural pulsations of the earth's magnetic field produce noise with a magnitude typically ranging from 10.sup.5 to 10.sup.8 fT. Noise fields of this magnitude tend to overwhelm the small signals of interest, making even the most sophisticated digital data processing techniques extremely difficult.
The use of superconducting sensor coil systems in conjunction with SQUID detectors is well known. These sensor coils typically have inductances of 1 or 2 microhenries in order to match the inputs of rf or dc SQUIDs. Typically, SQUID systems are arranged in either the magnetometer mode (single oriented sensor coil), the first-order gradiometer mode (two coils in either axial or planar arrangement), or the second-order gradiometer mode (two first-order gradiometers connected in series opposition).
The conventional attempt at solving the swamping problem is the use of superconducting gradiometry mode. With gradiometry, as stated above, sensor coils for the SQUID involve a pair of superconducting coils connected in series opposition in an attempt to electrically cancel the input of extraneous magnetic noise. However, unavoidable structural imperfections can limit the degree of cancellation obtainable.
When these gradiometers are used inside expensive magnetically shielded rooms, significant signal to noise ratio improvement can be realized. Even with this expensive configuration, however, noise in the 0.1 to 100 Hz range can sometimes interfere with SQUID magnetometer measurements.
An important point concerning the interfering magnetic noise is that its sources are distant with respect to the magnetic signals of interest. This means that the magnetic noise lines are substantially parallel as they approach the SQUID magnetometer.
The present invention provides apparatus for greatly improving and simplifying SQUID magnetometry detection through the use of superconducting surfaces used in conjunction with magnetometry and gradiometry coils. This configuration significantly reduces noise pick up while allowing SQUID measurements to be conducted in unshielded rooms, such as a hospital rooms and doctors' offices.
It is therefore an object of the present invention to provide apparatus for inhibiting noise pick up by SQUID magnetometers and gradiometers.
It is a further object of the present invention to provide SQUID magnetometry and gradiometry sensor apparatus that will operate successfully in an area which is not magnetically shielded.
It is a further object of the present invention to provide SQUID magnetometry and gradiometry sensor apparatus that will measure brain electrical activity over the entire skull area of a patient.
It is a still further object of the present invention to provide SQUID magnetometry and gradiometry sensor apparatus for detecting defects in materials.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.